[key to replace] is the character or keyname of the key you wish to replace
[replace with key] is the character or keyname of the key you want to replace it with
[how often] is a number that specifies how many seconds you want to replace the keystroke. Use 0 for always.

"C:\replace keys\replacekeys.exe" a TAB 0

This would replace the "a" keypress with the TAB keypress all of the time

"C:\replace keys\replacekeys.exe" s a 45

This would replace the "s" keypress with the "a" keypress no less than every 45 seconds